Laser diode pumped solid state laser with miniaturized laser head

ABSTRACT

A compact laser head for a solid state laser has a miniaturized laser rod and output coupling mirror which form a miniaturized laser cavity. A miniaturized frequency doubler crystal placed in the cavity provides frequency doubled output. The laser head is connected by an optical fiber to a separate power supply which contains a laser diode pumping source. A quick disconnect connector enables the fiber optic to be easily connected to the laser head. Pumping radiation is transmitted through the optical fiber to longitudinally end pump the laser rod using fiber coupling imagery. The fiber is aligned with the rod by the connector and the pumping radiation is imaged into the rod by a focussing sphere. The pumping volume is matched to the lasing volume which is determined by the cavity geometry. The quick disconnect laser head allows interchange of different heads with different output characteristics while using a single power supply.

RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 864,928 filed May19, 1986.

BACKGROUND OF THE INVENTION

The invention relates generally to solid state lasers such as Nd:YAGlasers and more particularly to compact packaging of solid state lasers.

U.S. patent application Ser. No. 730,002, filed May 1, 1985, now U.S.Pat. No. 4,653,056 issued Mar. 24, 1987, and CIP application Ser. No.811,546, filed Dec. 19, 1985, now U.S. Pat. No. 4,656,635 issued Apr. 7,1987, described solid state lasers which included a laser rod end pumpedby a laser diode. The pumping volume of the laser diode was matched tothe laser rod to optimize pumping efficiency and the laser cavity wasconfigured to provide a beam waist within the cavity at which afrequency doubler crystal could be placed. The laser diode was packagedin the same assembly. Each laser was designed to produce a particularoutput frequency determined by the material of the laser rod and thepresence or absence of a doubler crystal. However, for the widestvariety of applications and for the greatest ease of use, it isdesirable to have a laser with the most compact packaging possible and alaser with interchangeable components so that a number of differentoutput characteristics would be available from the same laser system.Since the output characteristics are largely determined by the designand components of the laser cavity, it is desirable to have a compactlaser head which is a separate unit from the rest of the laser systemand which can be readily coupled and decoupled to the rest of thesystem. Thus laser heads producing different output characteristics canbe readily substituted. It is also desirable to end pump the laser rod.

U.S. Pat. No. 4,387,297 issued June 7, 1983 to Swartz et al. and U.S.Pat. No. 4,409,470 issued Oct. 11, 1983 to Shepard et al. disclose ahand held typically gun-shaped laser-tube based laser scanning head. Thehead may also be streamlined or box-shaped. The head typically has avolume of 50-100 cubic inches and weights 1-2 pounds. The use of asemiconouctor laser diode in place of a He-Ne laser tube allows thelower sizes of the indicated ranges to be achieved. Power supplies,scanning motors and mirrors, and other circuitry are all included in thescanner head. The head is coupled to other components such as computerand data storage circuitry through an electrical cable.

U.S. Pat. No. 4,383,318 issued May 10, 1983 to Barry et al. shows alaser pumping system in which optic fibers in a fan-in arrangementconcentrate energy from an array of LED's or diode lasers to pointsalong the length of a laser rod.

U.S. Pat. No. 4,035,742 issued July 12, 1977 to Schiffner shows a devicefor optically pumping solid state lasers having a waveguide between thepumping source and laser rod disposed at an angle to the surface of therod determined by the index of refraction of the waveguide.

U.S. Pat. No. 3,982,201 issued Sept. 21, 1976 to Rosenkrantz et al.shows an end pumped solid state laser in which a diode laser array ispulsed at a rate and duty cycle to produce Cw operation.

SUMMARY OF THE INVENTION

Accordingly it is an object of the invention to provide a solid statelaser having a miniaturized laser head.

It is also an object of tne invention to provide a compact solid statelaser head which may be easily connected or disconnected from a laserdiode pumping source.

It is a furtner object to provide a compact solid state laser head whichis longitudinally end pumped.

It is another object of the invention to provide a solid state lasersystem with readily interchangeable laser heads.

The invention is a laser diode pumped solid state laser having compactpackaging with a miniaturized laser head coupled through a fiber opticto a power supply which includes a laser diode. The laser head containsa laser rod mounted in a housing with optical components to define alaser cavity and provide output coupling. A quick disconnect couplingfor the fiber optic to the laser head is provided and the laser headcontains an imaging lens to image tne output of the fiber optic into thelaser rod to longitudinally end pump the laser rod. The fiber opticallows the laser rod to be end pumped by the laser diode in a separatepower supply by a pumping scheme using fiber coupling imagery. The laserhead housing is made as small as possible and all components therein areminiaturized. The use of particular mounting means for the components,in particular ball and tube mounts, allows the use of very smallcomponents and the least amount of space. The components are positionedto match the pumping volume of the laser diode pumping source which istransmitted to the laser head through an optic fiber and imaged into thelaser rod to the lasing volume of the rod. By position and geometry ofthe optical elements defining the laser cavity a desired beam profile inthe cavity can be produced, which according to one aspect of theinvention, are used to provide TEM₀₀ output. A frequency doubler crystalcan also be mounted in the laser head in the optical cavity, preferablyat a beam waist, to provide frequency doubled output. In accordance withthe invention, various quick disconnect heads are readilyinterchangeable and operable with a single power supply which includes alaser diode pumping source. Each head can be designed to provideparticular output characteristics. Thus a very versatile system isprovided in which only the laser heads are interchanged. The small sizeof the laser head and the ability to move the laser head a distance fromthe power supply are highly advantageous for a variety of applications.Furthermore, the laser diode can be replaced when necessary without anyadjustment or realignment of the laser head components.

In accordance with the invention the laser diode pumping sources used topump the miniaturized laser heads are laser diode arrays and high powermulti-spatial mode extended emitter laser diodes.

In addition to the quick disconnect coupling at the laser head, a fixedcoupling could alternatively be utilized at the laser head; a quickdisconnect coupling could then be used at the power supply so that theoptical fiber could be disconnected from the supply rather than from thelaser head.

In addition to the use of imaging means for imaging the output of thefiber into the laser rod, the connector can hold the end of the fiber indirect contact with the end of the laser rod so that the absorptionregion of the pumping radiation in the rod matches the mode volume inthe laser cavity.

The laser cavity can be formed in the most compact manner by partiallymirrored surfaces on the ends of the laser rod, with one surface formingthe output coupler.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a sectional view of a quick disconnect miniaturized laser headaccording to the invention.

FIG. 2 is a sectional view of a ball and tube mount holding a doublercrystal in the laser head.

FIG. 3 is a perspective view of a laser system showing the laser headconnected to a power supply through a fiber optic.

FIG. 4 is a sectional view of a miniaturized laser head with resonatorhousing with deformable end portions.

FIG. 5 is a sectional view of the pumping volume mode volume match for afiber contacting the rod.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A quick disconnect compact solid state laser head 10 in accordance withthe invention is shown in FIG. 1. The laser head 10 has a hollow housing12 which is preferably substantially cylindrical or tubular andtypically made of stainless steel. At one end of housing 12 is end cap14, typically made of plastic, e.g. teflon impregnated Delrin, whichscrews onto or is otherwise attached to housing 12. A mirror 16 ismounted at the end of housing 12 and inside end cap 14. Mirror 16preferably has a concave inner surface and substantially flat outersurface. Mirror 16 forms a part of the laser optical cavity and is theoutput coupler for the laser cavity. Mirror 16 is held in a ball mount18 which is rotatably mounted in end cap 14 between beveled edge 20 ofhousing 12 and beveled edge 22 of end cap 14. Ball mount 18 has a hollowtube 24 extending therefrom into the interior of housing 12. Set screws26 extend through housing 12 and contact tube 24 so that the angularposition of ball mount 18 can be adjusted; there are typically three orfour set screws 26 spaced around the circumference of the housing.

Near the opposite end of housing 12 is mounted a solid state laser rod28 which is held in a holder or mount 30 which fits within the housing12; the rod 28 may be held in place by a set screw (not shown) whichalso stresses the rod to polarize the output. Alternatively, laser rod28 can be mounted in a ball mount if desirable to adjust its angularorientation. Mount 30 also holds a spherical lens or focussing sphere 32in a spaced relationship to laser rod 28; the lens 32 may be epoxied inplace. Spherical lens 32 is mounted against beveled edge 34 in endportion 36 of mount 30; end portion 36 is wider than the portion ofmount 30 which holds laser rod 28. An end cap 38 is placed at the end ofhousing 12 and contains the end portion 36 of mount 30. End cap 38 istypically made of teflon impregnated Delrin. End cap 38 also containscoupling means 40 which allow an optical fiber 42 to be connected tolaser head 10. Coupling means 40 is preferably a standard fiber opticconnector, either bayonet type or SMA (screw-on) type, e.g. Amphenol 905and 906 series connectors from Allied Corp., or any other coupling meanswhich provides fiber alignment and quick connect/disconnect. Couplingmeans 40 holds optical fiber 42 so that its end 44 is in close proximityto spherical lens 32. Laser rod 28, spherical lens 32 and the end 44 ofoptical fiber 42 are positioned so that the output of optical fiber 42is imaged into laser rod 28 to provide efficient longitudinal endpumping of laser rod 28. Coupling means 40 provides proper alignment offiber 42 which is reliable each time the fiber is connected to the laserhead.

A frequency doubler crystal 46 may also be mounted in the housing 12 inorder to produce a frequency doubled output. Doubler crystal 46 ismounted in a ball mount 48 which is held against beveled edge 50 on theinterior of housing 12 by ball retainer ring 52 which is spring loadedby spring 54 which is held by spring retainer 56 which is mounted inhousing 12. Ball mount 48 has a hollow tube 58 extending therefromlongitudinally in housing 12. Set screws 60 extend through housing 12and contact tube 58 so that the angular position of ball mount 48 can beadjusted; typically three or four set screws 60 are used.

In accordance with the principles described in U.S. patent applicationSer. No. 730,002, filed May 1, 1985, and CIP application Ser. No.811,546, filed Dec. 19, 1985, which are herein incorporated byreference, and the packaging techniques of tne present invention a veryshort optical cavity is produced. The optical cavity is defined bysurface 62 of mirror 16 and surface 64 of laser rod 28. Surface 64 istransmissive to pumping radiation but reflective to the lasing output oflaser rod 28 and the frequency doubled radiation in cases where thedoubler crystal 48 is used. By proper selection of the curvature of theoptical surfaces and the distances between the optical surfaces, thebeam profile within the cavity is controlled. In particular a beam waistis formed within the cavity which provides the optimal position forplacement of the doubler crystal 46. Also by mode matching the beamprofile to the cavity dimensions single transverse mode operation, e.g.,TEM₀₀ mode, can be achieved.

The optical elements 16, 28, 46 are provided in housing 12 at theappropriate positions according to a particular cavity design. Theelements are centered along the bore of housing 12. To perform theinitial alignment of optical elements 16 and 46, ball mounts 18 and 48,respectively, are rotated. The angular adJustment of doubler crystal 46in ball mount 48 is illustrated in FIG. 2. The crystal 46 is mounted ina channel through ball mount 48. Ball mount 48, typically made of teflonimpregnated aluminum, is rotatably held between beveled edge 50 ofhousing 12 and retaining ring 52. Tube 58 projects from ball mount 48into the bore of housing 12. A plurality of set screws 60, typicallythree or four, extend through the housing 12 and contact tube 58. ByadJustment of set screws 60, the tube 58 can be oriented in differentpositions, as illustrated, thereby rotating attached ball mount 48 andchanging the orientation of crystal 46. These ball mounts provide a verycompact configuration and ease of alignment; a ball mount could be usedfor the laser rod.

A significant feature of the invention is the longitudinal pumpingscheme using fiber coupling imagery. A view of the overall laser system66 is shown in FIG. 3 in which laser head 10 is coupled by optical fiber42 to a power supply 68. Power supply 68 contains a laser diode pumpingsource which is suitab1e for pumping the solid state laser rod in laserhead 10. The pumping radiation is transmitted from power supply 68 tolaser head 10 by optical fiber 42. As shown in FIG. 1, the pumpingradiation transmitted through optical fiber 42 is imaged by sphericallens 34 onto the end face 64 of laser rod 28. In accordance with theinvention the image size from the fiber is matched to the mode size inthe laser rod. The image size from the fiber is determined by the fiberdiameter and divergence of the light from the fiber. The distances fromthe spherical lens to the fiber and to the laser rod determine theimaging ratio. A spherical lens (focussing sphere) is preferred for itsease of centration in housing 12 and for its lack of alignment problems.The lasing volume in the rod is determined by the cavity configuration,i.e., the length of the cavity and the curvature of the output couplermirror and front surface of the laser rod. Thus for any desired cavityconfiguration, the pumping radiation from the fiber optic can be imagedinto the desired lasing volume of the rod for the most efficientoperation. The use of the fiber optic coupling allows the laser head tobe very compact and contain only the optical elements while all theelectronic and other elements, including the pumping source, can beplaced in a separate, stationary power supply. Since the optical fibercan be quite long, this system configuration provides great flexibilityin the use of the laser, making the laser head highly portable. Alsobecause of the quick disconnect feature, different laser head can bereadily interchanged. Thus a variety of different laser heads which havedifferent output characteristics can be used, essentially giving theuser the benefit of several different systems but without the expenseand redundancy of entire separate systems since only a new laser head isrequired with the same power supply to have an entire new system. Sincethe laser head contains only the optical components, the availability ofdifferent outputs becomes relatively economic. Also down time in thecase of a laser head failure is minimized since a replacement head caneasily be substituted. A further advantage to the use of fiber opticcoupling imagery for pumping the rod is that in the event that thepumping source must be replaced, the laser diodes can be easily replacedand matched into the fibers without need for realignment of the laserhead since the imaging of the fiber into the rod is not affected.

As an illustrative embodiment of the invention, a preferred laser headconfiguration is about 8.4 cm long and about 1.0 cm in diameter. Thelaser rod is a Nd:YAG crystal which is about 5 mm long and 3 mm indiameter. The spherical lens is 5 mm diameter; there is a space of about1.8 mm between between the end of the fiber and the spherical lens and aspace of about 3 mm from the lens to the end of the laser rod. Thedoubler crystal is a KTP crystal about 5 mm by 3 mm by 3 mm; the doublercrystal is 2.2 cm from the laser rod and 3.1 cm from the output couplermirror. A number of different optical fibers can be used; the smallerthe fiber, the higher the brightness, but the greater difficulty inalignment. A 200 micron diameter fiber, e.g., NRC FC-PC, a 125 microndiameter fiber, e.g. Corning 1504, and a 100 micron diameter fiber,e.g., NRC FC-MLD, all available from Newport Research Corporation,Fountain Valley, Calif., can be used. In a particular embodiment, a 200micron fiber is used with 1:1 imaging to produce a 200 micron diametermode volume in a 3 mm diameter Nd:YAG laser rod. By mode matching modesize is then 200 micron, and only TEM₀₀ output is obtained. Theprinciples of the invention can be applied to form even smaller laserheads, as small as 4 cm length and 7 mm diameter. Laser rods withlengths of about 1 mm and doubler crystals with lengths of about 2 mmcan be used.

A significant aspect of the invention is the laser diode pumping schemeusing fiber coupling imagery between a laser diode pumping source andthe laser rod in the miniaturized laser head for efficiently end pumpingthe laser rod while matching the laser mode volume to the pumpingvolume. Ideally a single spatial mode laser diode (single emitter) couldbe used; however, in practice these laser diodes are too low power.Accordingly laser diode arrays in which a plurality of diode emitters(typically 10 or more) are phase locked together are a preferred source.These laser diode arrays are described in the previously referencedpatent applications, and are illustrated by gallium aluminum arsenide(GaAlAs) laser diode arrays Model No. 2410 manufactured by Spectra DiodeLabs, San Jose, Calif. The higher power of the array is sufficient,despite the multi-spatial mode output, since the array beam can besuitably focused through the fiber and into the laser rod. Now there isa new type of diode available, a multi-spatial mode extended emitterlaser diode which has a wide or broad emitting stripe or active area andproduces a high power multi-spatial mode output. These single broadstripe or extended emitter laser diodes are exemplified by Sony Corp.Model Nos. SLD 301V/W having output power of 100 mW; SLD 302 V/W (200mW); SLD 303V/W (500 mW); SLD 304 V/W (1000 mW), as described in SonyNews and Information Bulletin No. 87S-013, 1987, and Sony SemiconductorData Book 1987 AlGaAs Laser Diodes, which are herein incorporated byreference. These diodes are compact and high efficiency and have atunable wavelength in the range 770-840 nm. Although there is adivergence problem and the far field beam pattern is unsuitable becauseof the multi-spatial mode output, it is possible to suitably focus thenear filed paatern into the fiber and to the laser rod to end pump thelaser rod and match the TEMOO mode of the laser cavity. Thus thesemulti-spatial mode extended emitter laser diodes form another preferredpumping source according to the invention.

The invention has been described with reference to a preferredembodiment having a laser cavity defined by a separate output couplingmirror and a mirrored end surface of the laser rod, as shown in FIG. 1,but other preferred cavity configurations are possible. In order to makethe shortest cavity possible a monolithic cavity can be used in whichsurfaces 64 and 65 of laser rod 28 form the laser cavity. Surface 64 ishighly transmissive to pump radiation and highly reflective to laserradiation; surface 65 is partly transmissive to laser radiation toprovide output coupling. The shape of surface 65 affects the cavity modevolume. Thus mirror 22 can be eliminated along with that part of thelaser head 10; if the frequency doubler (which now would be outside thecavity) is not used, the laser head can also be made smaller.

Other mounting means than the ball and tube previously described canalso be used. As illustrated in FIG. 4, a laser head 70 includes asubstantially tubular resonator housing 71 mounted in laser head housing71. Resonator housing 71 has a pair of end portions 73, 75 which extendout from narrow neck portions 74, 76. Output coupling mirror 77 ismounted in end portion 75 while solid state laser rod 78 and cavity endmirror 79 are mounted in end portion 73. Mirrors 77 and 79 define thelaser cavity; by bending the narrow portions 74, 76 of housing 71optical alignment of mirrors 77, 79 is performed. Mirror 79 can beseparate or formed on the end surface of rod 78. Other elements such asQ-switch 84 can be placed in housing 71.

Housing 72 contains fiber connector 80 which couples optical fiber 81 tohousing 72 to logitudinally end pump laser rod 78 from a remote laserdiode pump source. Connector 80 maintains fiber 81 in the properrelationship with any focusing optics 82 mounted in housing 72 bymounting means 83. The optics 82 and position of mounting means 83 aswell as fiber 81 control the focusing of the pumping radiation into therod. Connector 80 can provide quick disconnect as previously describedwhen it is desired to interchange laser heads; however, it is alsopossible to use a fixed connector 80. It is then possible to use a quickdisconnect connector 67 at the power supply so the fiber stays connectedto the laser head. Connector 80 can also hold fiber 81 in direct contactwith laser rod 78 as shown in FIG. 5. within the laser cavity eachspatial mode occupies a certain mode volume 84; TEM00 is preferred. Thepumping volume is matched to the mode volume. Pump radiation divergesinto a cone 85 determined by the fiber diameter and numerical aperture.Most of the pump radiation is absorbed into an absorption region 86 nearend face 87 of rod 78. By matching the mode volume 84 with theabsorption region 86, maximum absorption of the pump wavelength occurswithin the laser mode volume, maximizing coupling efficiency of the pumplight and optimizing laser operation in the desired mode (TEM00).Similarly, the use of focusing optics allows the matching of theresulting absorption area for the focused image to the mode volume.

The priciples of the invention apply to any diode pumped solid statelaser medium, including Nd:YAG, Nd:YLF, Nd:YAP, Nd:YALO, Nd:YVO₄,Nd:glass, Nd:BEL and Nd:GSGG.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

We claim:
 1. A compact laser head, comprising:a compact hollow housing;a miniaturized solid state laser rod mounted in the housing; lasercavity means defining a miniaturized laser cavity mounted in the housingwith the rod in the cavity, the laser cavity means including outputcoupler means; precision optical alignment connector means mounted tothe housing so that a region of absorption of pumping radiationtransmitted through the optical fiber is matched to a lasing beam volumeof the laser cavity so as to maximize absorption of said pump radiationin said laser beam volume.
 2. The laser head of claim 1 furtherincluding imaging means mounted in the housing for imaging the output ofthe optical fiber into the laser rod.
 3. The laser head of claim 2further including means for matching an image size of the optical fiberto a mode size of the laser rod.
 4. The laser head of claim 3 whereinthe means for matching the fiber image size to the mode size includesbeam shaping means and positioning means for the imaging means.
 5. Thelaser head of claim 1 further including a miniaturized frequency doublercrystal mounted in the housing in the miniaturized laser cavity.
 6. Thelaser head of claim 1 wherein the laser cavity means comprise partiallymirrored surfaces on the front and back ends of the laser rod, the frontmirrored surface providing the output coupler means.
 7. The laser headof claim 1 further including a miniaturized Q-switch operatively toproduce a pulsed output.
 8. The laser head of claim 1 wherein theconnector means connects an optical fiber to the housing with an end ofthe optical fiber in contact with an end of the laser rod.
 9. The laserhead of claim 1 wherein the laser rod is selected from Nd:YAG, Nd:YLF,Nd:YAP, Nd:YALO, Nd:YVO₄, Nd:glass, Nd:BEL, Nd:GSGG.
 10. A solid statelaser system comprising:a compact laser head of claim 1; a laser powersupply including a laser diode pumping source; an optical fiberconnected from the power supply to the laser head to transmit pumpingradiation from the pumping source to the laser head.
 11. The lasersystem of claim 10 wherein the laser diode pumping source is amulti-spatial mode extended emitter laser diode.
 12. The laser system ofclaim 10 wherein the laser diode pumping source is a laser diode array.13. The laser system of claim 10 wherein the connector means providesquick disconnect of the optical fiber from the housing.
 14. The lasersystem of claim 10 further including means to quickly disconnect theoptical fiber from the power supply.
 15. The laser system of claim 10further including imaging means mounted in the housing for imaging theoutput of the optical fiber into the laser rod.
 16. The laser system ofclaim 15 further including means for matching an image size of theoptical fiber to a mode size of the laser rod.
 17. The laser system ofclaim 10 further including a miniaturized frequency doubler crystalmounted in the housing in the miniaturized laser cavity.
 18. The lasersystem of claim 10 wherein the laser cavity means comprise partiallymirrored surfaces on the front and back ends of the laser rod, the frontmirrored surface providing the output coupler means.
 19. The lasersystem of claim 16 wherein the means for matching the fiber image sizeto the mode size includes beam shaping means associated with the cavityand positioning means for the imaging means.
 20. The laser system ofclaim 10 wherein the connector means connects an optical fiber to thehousing with an end of the optical fiber in contact with an end of thelaser rod.